1. Field of the Invention
The present invention relates to a photo detecting device used in an optical pickup for an optical disk apparatus and the like and, more specifically, to an improvement of a photo detecting device including a photoelectric conversion device and the signal processing circuit formed on the same semiconductor chip.
2. Description of the Background Art
An optical pickup used in a conventional optical disk apparatus comprises a photoelectric conversion device formed on a semiconductor chip for receiving light reflected from a recording medium to convert the same into electric signals. The electric signals outputted from such a photoelectric conversion device are subjected to processes such as arithmetic amplification in a signal processing circuit, not formed as part of the semiconductor chip, to be converted into information reproducing signals, focus error signals and tracking error signals.
A recently developed integrated circuits technique involves an OPIC (Optical Integrated Circuit) photo detecting device in which the signal processing circuit as well as the photoelectric conversion device are formed on one semiconductor chip, which results in simplification of the design of the signal processing circuit.
FIG. 4 shows one example of the conventional OPIC photo detecting device. The photo detecting device comprises three photoelectric converting elements 11, 12 and 13. The central photoelectric converting element 11 has four divided light receiving regions 11a, 11b, 11c and 11d. The output signals from the light receiving regions 11a to 11b are connected to minus input terminals of differential amplifiers A.sub.11, A.sub.12, A.sub.13 and A.sub.14, respectively, and to an input terminal of an amplifier A.sub.15 through coupling capacitors C.sub.11, C.sub.12, C.sub.13 and C.sub.14. The differential amplifiers A.sub.11 to A.sub.14 have their plus input terminals all grounded.
The photo electric converting elements 12 and 13 on both ends are connected to minus input terminals of the differential amplifiers A.sub.16 and A.sub.17, respectively. The differential amplifiers A.sub.16 and A.sub.17 have their plus input terminals grounded.
In the photo detecting device of FIG. 4, signals Sa, Sb, Sc and Sd outputted from the light receiving regions 11a to 11d are amplified by the differential amplifiers A.sub.11 to A.sub.14, respectively, to be externally outputted. In an external circuit (not shown), the following arithmetic operation is carried on the signals Sa to Sd to provide a focus error signal FES. EQU FES=(Sa+SC)-(Sb+Sd)
The signals Sa to Sd outputted from the light receiving regions 11a to 11d have their fluctuation of low frequency components removed through coupling capacitors C.sub.11 to C.sub.14, and thereafter they are added to each other. A reproduction signal RF is provided by amplifying the summed signal (Sa+Sb+Sc+Sd) by the amplifier A.sub.15.
Further, signals Se and Sf outputted from the photoelectric converting elements 12 and 13 on both ends are amplified by the differential amplifiers A.sub.16 and A.sub.17, respectively, to be externally outputted, and the tracking error signal TES is provided by a subtracting operation by an external circuit.
In the photo detecting device of FIG. 4, a plurality of operational amplifiers must be provided in the external circuit for obtaining the focus error signal FES. It is difficult to provide the same characteristics to all of these plurality of operational amplifiers in the external circuit. Accordingly, when the ambient temperature changes, an offset voltage tends to be generated in the focus error signal FES obtained from the external circuit at the in-focus state. The offset voltage prevents accurate focus control.
In addition, since the photoelectric converting element 11 at the center comprises four light receiving regions 11a to 11d, four output terminals are necessary to output signals Sa to Sd to the outside from the light receiving regions. This increases the number of output terminals of the photo detecting device itself, which makes it difficult to reduce the size of the photo detecting device.,
In addition, when the circuit elements are to be formed on an IC chip, a capacitor requires relatively large area. Therefore, when four coupling capacitors C.sub.11 to C.sub.14 are provided, these capacitors occupy 1/8 to 1/4 of the whole area of the IC chip, which naturally results in the IC chip being larger. This further prevents reduction in size of the photo detecting device.